The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark ignition engines introduce a fuel/air mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (hereafter ‘TDC’) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
An engine configured for spark ignition can be adapted to operate in a homogeneous charge compression ignition (hereafter ‘HCCI’) mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The controlled auto-ignition combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI mode has an intake charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute fuel/air mixture, i.e., lean of a fuel/air stoichiometric point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous fuel/air mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
When an engine operates in the HCCI mode, the engine control comprises lean air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. When the engine operates in the spark-ignition combustion mode, the engine control comprises stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake air flow to achieve the stoichiometric air/fuel ratio. It is known that combustion in each cylinder can vary significantly due to differences in individual fuel injector characteristics and other factors in a multi-cylinder HCCI engine.
Known engine valve control strategies comprise an exhaust recompression strategy to control the cylinder charge temperature by trapping hot residual gas from the previous combustion cycle. This can include advancing closing timing of the exhaust valve and correspondingly delaying opening timing of the intake valve, preferably symmetrical to the exhaust valve closing timing about TDC of the intake stroke to create a negative valve overlap (hereafter ‘NVO’) period. The NVO period is defined as a crank angle duration between the exhaust valve closing and the intake valve opening. The cylinder charge composition and temperature are affected by the exhaust valve closing timing. In particular, greater amounts of hot residual gas from the previous combustion cycle can be retained with the advanced closing of the exhaust valve, reducing incoming fresh air mass into the cylinder, resulting in increased cylinder charge temperature and lower cylinder charge oxygen concentration.
It is known to couple the engine valve control strategy and a fuel injection strategy to stabilize combustion. For example, at a low fueling rate, temperature of the cylinder charge may preclude achieving stable auto-ignited combustion of the cylinder charge regardless of the NVO period. It is known to increase temperature of the cylinder charge by pre-injecting fuel into the combustion chamber, preferably during the NVO period. A portion of the pre-injected fuel reforms due to pressure and temperature during recompression, releasing heat energy and increasing the cylinder charge temperature to achieve auto-ignited combustion of the cylinder charge created by a subsequent main fueling during the compression stroke. It is known that the amount of auto-thermal fuel reforming is based upon mass and timing of the pre-injection fueling, with the fuel reforming increasing with earlier timing of the pre-injection fueling and/or greater fuel mass of the pre-injection fueling. It is known that an amount of fuel reforming that is greater than required for stable combustion can increase brake-specific fuel consumption, and that an amount of fuel reforming that is less than required for stable combustion can increase combustion instability. Fuel reforming can vary due to injection timing, trapped exhaust gas temperature and pressure, and other factors.